✨[Feature] Route FP16 Layer Norm properly

[narendasan]

Is your feature request related to a problem? Please describe.

We now see these warnings:

WARNING:torch_tensorrt [TensorRT Conversion Context]:Detected layernorm nodes in FP16.
WARNING:torch_tensorrt [TensorRT Conversion Context]:Running layernorm after self-attention with FP16 Reduce or Pow may cause overflow. Forcing Reduce or Pow Layers in FP32 precision, or exporting the model to use INormalizationLayer (available with ONNX opset >= 17) can help preserving accuracy.

Describe the solution you'd like

Add a lowering pass to detect this case and properly route the subgraph to the right converters

Describe alternatives you've considered

Additional context